Copper is widely used in the semiconductor industry, but copper oxidation is a critical issue and needs to be prevented. In the assembly process, Cu is used to connect chip to chip, chip to substrate as Flip Chip bump or wire for wire bonding, etc. Generally, a flux agent is added as stand-alone material or embedded in the underfill to remove Cu oxidation during assembly. The use of a flux is however not preferred due to the flux residues that are hard to remove and generate reliability issues.
A known solution to prevent copper oxidation is the use of graphene. Graphene is a 2-dimensional hexagonal ring structure which can act as a membrane to prevent oxygen molecules from penetrating into a copper surface, therefore avoiding the copper oxidation reaction. Graphene consists of sp2-bonded carbon atoms with bond length approximately 1.42 angstroms smaller than copper (2.55 Å) and oxygen molecules (3.46 Å).
It is known to transfer a sheet of graphene by stamp techniques, as shown for example in ‘Site-specific transfer-printing of individual graphene microscale patterns to arbitrary surfaces’, Ya-Qing Bie et al, Advanced Materials 2011, 23, 3938-3943. This technique however requires a patterning step to create micro-stamps of graphene on PMMA supports and a complex transfer process involving a glass fiber controlled by a micromanipulator. Transferring larger graphene sheets to a surface is also known, as illustrated for example in ‘A review of chemical vapor deposition of graphene on copper’, Mattevi et al, Journal of Materials Chemistry, 2011, 21, 3324-3334. In the latter publication, transfer methods are described for transferring a sheet of graphene grown by CVD on a copper substrate, by attaching the sheet to a carrier (e.g. PMMA) and etching away the copper. The carrier allows deposition of the sheet on a substrate, after which the carrier itself is removed, e.g. by dissolving with acetone in the case of PMMA. However, when applied to the construction of a single functional microdevice, this technique will cause considerable waste of graphene material as only a small portion of the sheet is used.